Method of forming multi-layered coating film

ABSTRACT

This invention provides a method for forming multi-layered coating film excelling in appearance, corrosion resistance and chipping resistance, which comprises applying a first coloring paint (B), second coloring paint (C) and clear paint (D) onto cured coating film of a specific electrodeposition paint (A) of low weight loss under heating, wet-on-wet by the order stated; and heat-curing the three-layered coating film simultaneously.

TECHNICAL FIELD

This invention relates to a method for forming multi-layered coatingfilm excelling in appearance, corrosion resistance and chippingresistance, which comprises applying a first coloring paint, secondcoloring paint and clear paint onto cured coating film of anelectrodeposition paint, wet-on-wet by the order stated; and heat-curingthe three-layered coating film simultaneously.

BACKGROUND ART

Coating of metallic shaped articles such as automobile bodies, metallicparts of two-wheeled vehicles, house electric appliances, steelfurnitures and the like is normally conducted by the steps of applyingan electrodeposition paint, baking the resulting coating film, applyingan intermediate coat thereon and baking the same, and then applying atop coat and baking the same to form a multi-layered coating film.However, when baking is performed after application of each of thosepaints, not only high energy costs are incurred for the baking but alsogreat labor and costs are required for operation and maintenance of thebaking facilities. Hence development of coating film-forming methodcapable of reducing use of volatile organic compounds and achievingenergy-saving and step-reduction has been in demand.

For example, JP Sho 61(1986)-141969A disclosed a metallic finishingmethod comprising applying onto a substrate an organic solvent-based ornon-aqueous dispersion type thermosetting paint, a thermosetting,water-based metallic paint and a transparent thermosetting paint, by theorder stated, and heating the three-layered coating film to cure themultiple layers simultaneously. This method, however, is subject to aproblem that once finishing quality of the electrocoated film which isthe undercoat applied before the three layers are formed thereondegrades, that of the multi-layered coating film surface also degradesas influenced thereby.

JP Hei 3(1991)-181369A disclosed a method for forming high qualitymulti-layered coating film by 3C1B process, comprising applying anintermediate coat, top base coat and top clear coat, by the orderstated, onto a laser-treated dull steel plate onto which an undercoathas been applied by cationic electrocoating and subsequently baked, atleast one of the three paints being a non-aqueous dispersion type paint;and baking the multi-layered coating film. This method, however, issubject to a problem that costly laser-treated dull steel plate-madeshaped articles must be used to secure good finishing quality ofperpendicular and horizontal parts, for forming a high qualitythree-layered coating film by single baking.

Furthermore, JP 2002-273322A disclosed a method for formingmulti-layered coating film having high decorative effect and visualappearance, comprising successively applying a first water-borne basepaint, second water-borne base paint and clear paint onto an object tobe coated and simultaneously heat-curing all the uncured coating films,the first water-borne base paint and/or the second water-borne basepaint containing effect pigment and the solid content of uncured firstbase paint coating film being 40-95% by weight. While this method canprovide multi-layered coating film of good appearance and free offoaming or pinholing, there remains a problem that the multi-layeredcoating film surface becomes uneven, as affected by unevenness on theelectrocoated film surface.

JP 2002-282773A disclosed a method of forming laminated coating film,comprising a step of forming an uncured intermediate coat by forming acationic electrocoated film and applying onto the cured electrocoatedfilm a water-borne intermediate paint; step of forming an uncured basecoating film by applying a water-borne base paint onto the uncuredintermediate coating film; a step of applying a clear paint on theuncured base coating film; and simultaneously heat-curing those uncuredintermediate coat, base coat and clear coat; said cationicelectrodeposition paint containing a binder resin which containscationic epoxy resin and blocked isocyanate curing agent, neutralizingacid, organic solvent and metallic catalyst. However, because the methodas described in the Official Patent Gazette uses, as the cationicelectrodeposition paint, a paint containing cationic epoxy resin andblocked isocyanate curing agent, undulation or unevenness result on theelectrocoated surface due to volatilization of the blocking agent duringthe baking for curing the electrocoated film, and when the multi-layeredcoating film is formed on such an electrocoated film surface by3-coat-1-bake system of the first coloring paint, second coloring paintand clear paint, the finally formed multi-layered coating film showsdegraded appearance, as affected by the unevenness of the electrocoatedfilm.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a method of formingmulti-layered coating film excelling in appearance, corrosionresistance, chipping resistance and so on, while coping with environmental problems by reducing generation of volatile organic compound incoating metallic objects.

We have engaged in concentrative studies to now discover that the aboveobject can be accomplished by successively applying a first coloringpaint, second coloring paint and clear paint onto cured coating filmsurface of specific electrodeposition paint of little weight loss underheating, and curing the three-layered coating film by single baking(which procedures may hereafter be referred to as 3-coat-1-bake or 3C1B)and completed the present invention.

Thus, the invention provides a method of forming multi-layered coatingfilm which comprises applying a first coloring paint (B), secondcoloring paint (C) and clear paint (D) successively wet-on-wet, onto acured coating film of an electrodeposition paint (A) showing a heat loss(X) of not more than 5% by weight, said heat loss being calculatedaccording to the following equation.Heat loss (X)=[(Y−Z)/Y]×100

-   -   [wherein Y is the weight of a dry coating film remaining after        removal of the water content from an uncured coating film, which        is obtained by electrocoating the electrodeposition paint (A),        by heating at 105° C. for 3 hours; and    -   Z is the weight of the cured coating film after heating the dry        coating film at 170° C. for 20 minutes].

According to the method of the present invention, coated articles withlittle generation of volatile organic compound and exhibiting excellentappearance, corrosion resistance, chipping resistance and so on can beobtained. Still in addition, because the method of the invention adopts3C1B system, it can contribute to energy saving and space reduction.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a model diagram showing frequency characteristics of a powerspectrum.

In FIG. 1, 1 shows short wavelength region, 2 shows middle wavelengthregion and 3, long wavelength region.

FIG. 2 shows frequency characteristics of power spectrum of anelectrodeposited coating film according to the present invention.

FIG. 3 shows frequency characteristics of power spectrum of aconventional electrodeposition coating film.

Hereinafter the method of forming multi-layered coating film of thepresent invention is explained in further details.

Substrate

The substrate or object to which the method of the present invention isapplicable is subject to no particular limitation so long as it is of anelectrocoatable material. As such materials, for example, metals such asstainless steel, iron, steel, copper, zinc, tin, aluminium, alumite andthe like; alloys of such metals, sheets which are plated with suchmetals and laminated sheets of such metals can be named, which can begiven surface treatment, primer treatment or the like where necessary,to be imparted with improved corrosion resistance and adherability. Forexample, stainless steel can be given a chromium surface treatment. Asthe coating object, automobile bodies are preferred, and the steel sheetto serve as the substrate can be advancedly zinc phosphate-treated ascustomarily practiced.

Electrodeposition Paint (A)

The above-described substrate is electrocoated with saidelectrodeposition paint (A). In the present invention, as theelectrodeposition paint (A), one having a heat loss (X)^((note 1)) ofits electrocoating film not more than 5 wt %, preferably not more than 4wt %, inter alia, not more than 3.5 wt %, is used. When anelectrodeposition paint forming an electrocoated film having a heat loss(X) more than 5 wt % is used, degradation in surface finish of 3C1Bmulti-layered coating film composed of the electrocoated film, firstcolored coating film, second colored coating film and clear coating filmmay take place, which is undesirable.

-   -   (Note 1) Heat loss (X) is calculated according to the following        equation:        Heat loss (X)=[(Y−Z)/Y]×100    -   [wherein Y is the weight of a dry coating film remaining after        removal of the water content from an uncured coating film, which        is obtained by electrocoating the electrodeposition paint (A) by        heating at 105° C. for 3 hours; and    -   Z is the weight of the cured film after heating the dry coating        film at 170° C. for 20 minutes].

Furthermore, the electrodeposition paint (A) to be used in the presentinvention is preferably such that it forms cured electrocoated filmhaving an average power spectral value (note 2) within a wavelengthregion of 0.02-1 mm of generally not higher than 70, in particular,20-49, inter alia, 33-45, and/or an integration value of the powerspectral values within the wavelength region of 0.02-1 mm of generallynot higher than 1.7×10⁵, in particular, 5×10⁴-1.2×10⁵, inter alia,8×10⁴-1.1×10⁵, said values being obtained by power spectrum frequencyanalyses which comprise measuring surface roughness of an electrocoatedfilm which has been cured at 170° C. for 20 minutes, over a measuringlength of 50 mm at 10 μm intervals with a surface roughness meter andthen Fourier transforming the so obtained measurement data.

FIG. 1 is a model diagram of the frequency characteristics of powerspectral values over a range less than 0.1 mm (short wavelength region),0.1-1 mm (middle wavelength region) and 1 mm-10 mm (long wavelengthregion).

FIG. 2 is the frequency characteristics diagram of a 20 μm-thick curedcoating film which is formed by electrocoating electrodeposition paintNo. 1 of the present invention as obtained in later appearing ProductionExample 16 and heating the film at 170° C. for 20 minutes; and FIG. 3shows the frequency characteristics diagram of a 20 μm-thick curedcoating film which is formed by electrocoating a conventionalelectrodeposition paint as obtained in later appearing ProductionExample 23 and heating the film at 170° C. for 20 minutes.

As is clear upon comparing FIG. 2 with FIG. 3, the electrodepositionpaint (A) used in the present invention forms cured electrocoated filmexcelling in power spectral values over from the short wavelength regionto middle wavelength region.

-   -   (Note 2) Power spectral values:        -   The values obtained by measuring roughness of cured surface            of an electrocoated film of an electrodeposition paint with            a surface roughness measuring instrument (SURFCOM 130A,            tradename, Tokyo Seimitsu Co., Ltd.) over a measuring length            of 50 mm at 10 μm-intervals, and Fourier transforming the            obtained data.

As the electrodeposition paint (A) to be used in the method of thepresent invention, particularly those comprising base resin (a) which isobtained by reacting epoxy resin (a₁), amine compound (a₂) and phenoliccompound (a₃); and epoxy resin (b) as crosslinking agent are preferred.

As the epoxy resin (a₁) used for the production of the base resin (a),particularly the epoxy resins having at least two epoxy-containingfunctional groups per molecule which are represented by the followingformula (1)

are preferred.

Said epoxy resin (a₁) can be those known per se, for example, thosewhich are described in JP Sho 60(1985)-170620A, JP Sho 62(1987)-135467A,JP Sho 60(1985)-166675A, JP Sho 60(1985)-161973A and JP Hei2(1990)-265975A can be used.

The epoxy resin (a₁) also includes those with their termini bonded toresidual groups of polymerization initiating component, i.e., activehydrogen-containing organic compound residues. As activehydrogen-containing organic compounds which are the precursors thereof,for example, alcohols such as aliphatic monohydric alcohol, aromaticmonohydric alcohol, aliphatic or alicyclic polyhydric alcohol and thelike; phenols; fatty acids; aliphatic, alicyclic or aromatic polybasicacids; oxy acid; polyvinyl alcohol, partial hydrolyzate of polyvinylacetate, starch, cellulose, cellulose acetate, cellulose acetatebutylate, hydroxyethyl cellulose, allylpolyol resin, styrene-allylalcohol copolymer resin, alkyd resin, polyester polyol resin,polycaprolactonepolyol resin and the like can be named. These activehydrogen-containing organic compounds may also have a skeletal structurein which unsaturated double bond is epoxidated, concurrently with theactive hydrogen.

The epoxy resin (a₁) can be prepared, for example, by conducting apolymerization reaction using above-described active hydrogen-containingorganic compound as the initiating agent, in the presence of4-vinylcyclohexene-1-oxide alone or concurrent presence therewith ofanother epoxy-containing compound, said polymerization being induced bythe epoxy groups contained in the named compounds, to form polyetherresin, and then epoxidating the vinyl groups present in its side chainswith oxidizing agent such as peracids or hydroperoxides.

4-Vinylcyclohexene-1-oxide can be prepared, for example, by partiallyepoxidating vinylcyclohexene, which is formed through dimerizationreaction of butadiene, with peracetic acid.

As other epoxy-containing compound copolymerizable therewith, anycompounds having epoxy groups can be used without particular limitation,while those containing one epoxy group per molecule are preferred fromthe standpoint of ease of production. More specifically, for example,oxides of unsaturated compounds such as ethylene oxide, propylene oxide,butylenes oxide, α-olefin epoxides represented by the following formula(2)

[in which n is an integer of 2-25], styrene oxide and the like; glycidylethers of compounds having hydroxyl groups such as allyl glycidyl ether,2-ethylhexyl glycidyl ether, methyl glycidyl ether, butyl glycidylether, phenyl glycidyl ether and the like; and glycidyl esters oforganic acid such as fatty acid.

The ring-opening (co)polymerization of epoxy groups in the presence of4-vinylcyclohexene-1-oxide alone or in concurrent presence of otherepoxy-containing compound is preferably conducted in the presence of acatalyst. As the catalyst, for example, amines such as methylamine,ethylamine, propylamine, piperazine and the like; organic bases such aspyridines, imidazoles and the like; organic acids such as formic acid,propionic acid and the like; inorganic acids such as sulfuric acid,hydrochloric acid and the like; alkalai metal alcoholates such as sodiummethylate and the like; alkalies such as KOH, NaOH and the like; Lewisacids such as BF₃SnCl₂, AlCl₃, SnCl₄ and the like and complexes thereof;and organometal compounds such as triethylaluminium, diethylzinc and thelike can be named.

Such catalyst can be used normally within a range of 0.001-10 wt %,preferably 0.1-5 wt %, to the reactants. The ring-opening(co)polymerization reaction can be conducted generally at temperaturesranging −70° C.-200° C., preferably −30° C.-100° C. This reaction ispreferably conducted in a solvent, and as the solvent ordinary organicsolvent having no active hydrogen can be used.

Thus obtained polyether resin (ring-opened (co)polymer) can then beconverted to an epoxy resin (a₁) having the functional groups of theformula (1), by epoxidating the vinyl groups (—CH═CH₂) directly bound tothe carbon atoms in the alicyclic structure of side chains thereof.

The epoxidation can be effected using peracids or hydroperoxidies. Asperacids, for example, performic acid, peracetic acid, perbenzoic acidtrifluoroperacetic acid and the like can be used, and as hydroperoxides,for example, hydrogen peroxide, tert-butyl peroxide, cumene peroxide andthe like can be used. The epoxidation reaction can be practiced in thepresence of a catalyst, where necessary.

The functional groups of the formula (1) are formed as the vinyl groupsin 4-vinylcyclohexene-1-oxide in the ring-opened (co)polymer areepoxidated. In this epoxidation reaction, where an alicyclicoxyrane-containing compound as afore-named is concurrently present asthe epoxy-containing compound, vinyl groups in said compound mayoccasionally be also epoxidated, however to result in a structuredifferent from the functional group of the formula (1).

Use or non-use of a solvent or the temperature of the epoxidationreaction can be suitably adjusted according to the apparatus orproperties of the starting materials used. Depending on the epoxidationreaction conditions, a substituent of the following formula (3)

in the starting material(s) and/or the substituent of the formula (1) asformed in the reaction may side-react with epoxidation agent used,simultaneously with the epoxidation of vinyl groups in the startingpolymer, to form modified substituents which come to be concurrentlypresent in the epoxy resin (a₁). The ratio of content of these modifiedsubstituents differs depending on the kind of epoxidation agent used,molar ratio between the epoxidation agent and the vinyl groups and thereaction conditions.

Commercial products may also be used as such epoxy resin (a₁), forexample, EHPE 3150 (tradename, Daicel Chemical Industries, Ltd.), inwhich vinyl groups in ring-opened polymer of 4-vinylcyclohexene-1-oxideare epoxidated, having an average degree of polymerization ranging15-25.

It is sufficient that at least two epoxy-containing functional groups ofthe formula (1) are present per molecule of the epoxy resin (a₁) whichcan generally have an epoxy equivalent within a range of 140-1,000,preferably 170-300, and a number-average molecular weight (note 3)generally within a range of 200-50,000, preferably 1,000.

-   -   (Note 3) number-average molecular weight:        -   A value determined by following JIS K0124-83, using as the            separation columns TSK GEL 4000 H_(XL)+G3000 H_(XL)+G2500            H_(XL)+G2000 H_(XL) (tradename, Tosoh Corporation) and as            the eluent tetrahydrofuran for GPC and measuring at 40° C.            at a flow rate of 1.0 ml/min.; and calculating referring to            the chromatogram obtained with RI refractometer and            polystyrene calibration line.

Amine compound (a₂) to be reacted with the epoxy resin (a₁) is acationic property-imparting component for introducing amino groups intothe epoxy resin (a₁) to cationize the same. As the amino compound (a₂),one having at least one hydrogen to react with epoxy group is used.

As such amine compound (a₂), for example, mono- or di-alkylamines suchas monomethylamine, dimethylamine, monoethylamine, diethylamine,monoisopropylamine, diisopropylamine, monobutylamine, dibutylamine andthe like; alkanolamines such as monoethanolamine, diethanolamine,mono(2-hydroxylpropyl)amine, di(2-hydroxypropyl)amine,tri(2-hydroxypropyl)amine, monomethylaminoethanol, monoethylaminoethanoland the like; alkylenepolyamines such as ethylenediamine,propylenediamine, butylenediamine, hexamethylenediamine,tetraethylenepentamine, pentaethylenehexamine, diethylaminopropylamine,diethylenetriamine, triethylenetetramine and the like and ketiminationproducts of these polyamines; alkyleneimines such as ethyleneimine,propyleneimine and the like; and cyclic amines such as piperazine,morpholine, pyrazine and the like can be named. Of these, primary orsecondary amine compounds having primary hydroxyl groups areparticularly preferred.

As the phenolic compound (a₃), one having at least one phenolic hydroxylgroup per molecule can be used. More specifically, for example,polyhydric phenolic compounds such as 2,2-bis(p-hydroxyphenyl)propane,4,4′-dihydroxybenzophenone, 1,1-bis(p-hydroxyphenyl)ethane,1,1-bis(p-hydroxyphenyl)isobutane,2,2-bis(4-hydroxy-3-tert-butylphenyl)propane,bis(2-hydroxynaphthyl)methane, 1,5-dihydroxynaphthalene,bis(2,4-dihydroxyphenyl)methane, 1,1,2,2-tetra(p-hydroxyphenyl)ethane,4,4-dihydroxydiphenyl ether, 4,4-dihydroxydiphenylsulfone, phenolnovolak, cresol novolak and the like can be named.

Monophenolic compounds such as phenol, nonylphenol, α- or β-naphthol,p-tert-octylphenol, o- or p-phenylphenol and the like may also be used.

For forming a coating film exhibiting better corrosion resistance, useof bisphenol compound such as bisphenol A[2,2-bis(p-hydroxyphenyl)propane], bisphenolF[bis(p-hydroxyphenyl)methane] and the like as the phenolic compound(a₃) is particularly preferred.

Of those bisphenol compounds, particularly those represented by thefollowing formula (4)

-   -   [in the formula, n is a number 0-8, and R₆ stands for a residue        of an active hydrogen compound]        having a number-average molecular weight of at least 200,        preferably about 800-about 3,000 and on the average not more        than 2, preferably 0.8-1.2, phenolic hydroxyl group(s) per        molecule are suitable.

As the active hydrogen-containing compound which is a precursor of R₆ inthe above formula (4), for example, compounds such as amines likesecondary amines; phenols like nonylphenol; organic acids such as fattyacid; thiols; alcohols such as alkyl alcohol, cellosolve,butylcellosolve, carbitol and the like; and inorganic acids can benamed. Of these, the most preferred are dialkanolamines which aresecondary amines having primary hydroxyl groups and monophenols such asnonylphenol, phenylphenol and phenol. In particular, use of a secondaryamine having primary hydroxyl group(s) improves curability.

The above formula (4) shows a structure in which R₆ ⁻ and —OH are boundrespectively to its two terminals, while it is permissible thatmolecules of the structure whose two terminals are bound to either R₆—or —OH alone are mixed.

Base resin (a) can be obtained by reacting above-described epoxy resin(a₁) with amine compound (a₂) and phenolic compound (a₃). Such a baseresin (a) is advantageous over conventional base resins whose basecomponent is bisphenol A type epoxy resin, in that the former excels incorrosion resistance and in electrodeposition coating ability foralloyed zinc-plated steel sheet.

The reaction ratio of the epoxy resin (a₁), amine compound (a₂) andphenolic compound (a₃) is subject to no particular limitation and can besuitably selected according to intended utility of the resulting resinfor paint. Whereas, it is generally preferred to use them at such aratio, per mol of the epoxy-containing functional group of epoxy resin(a₁), that the primary or secondary amino group in the amine compound(a₂) is within a range of 0.1-1 mol, in particular, 0.4-0.9 mol, and thephenolic hydroxyl group in the phenolic compound (a₃), within a range of0.02-0.4 mol, in particular, 0.1-0.3 mol; and to make the sum of abovemol numbers of the amine compound (a₂) and phenolic compound (a₃) fallwithin a range of 0.75-1.5 mols, in particular, 0.8-1.2 mols, per mol ofthe epoxy-containing functional group in the epoxy resin (a₁).

The reaction using these components can be conducted, for example, attemperatures ranging from 50-300° C., in particular, 70-200° C. Theorder in the reaction is not particularly limited. All of the componentsmay be simultaneously charged in a reactor to initiate the reaction, orthe components other than epoxy resin (a₁) can be added to the resin(a₁) by optional order to cause successive reactions.

The base resin (a) preferably has, in general terms, an amine valuewithin a range of 20-150 mgKOH/g, in particular, 35-100 mgKOH/g; ahydroxyl value within a range of 300-1,000 mgKOH/g, in particular,350-700 mgKOH/g; and a number-average molecular weight (cf. note 3)within a range of 800-15,000, in particular, 1,000.

The base resin (a) may further be reacted with a cationizing agentduring or after its preparation, where necessary. As the cationizingagent, for example, tertiary amines such as triethylamine,triethanolamine, N,N-dimethylethanolamine, N,N-methyldiethanolamine,N,N-diethylethanolamine, N-ethyldiethanolamine and the like can be used.They may be protonated with acid in advance, and reacted with epoxygroups to be converted to quaternary salts.

On the other hand, as the epoxy resin (b) to be used as the curing agentfor the base resin (a), polyepoxide compound having, on the average, atleast two epoxy-containing functional groups formed of alicyclicskeletal structure bound to epoxy group(s), and glycidyl etherifiednovolak resin can be named. Specifically, epoxy resins (b-1), (b-2) or(b-3) having the specific structures explained hereinafter arepreferred.

Epoxy Resin (b-1):

Polyepoxide compounds having recurring units of the following formula(5)

more specifically, including those as explained in relation to the epoxyresin (a₁). As commercial products, EHPE3150 (tradename, Daicel ChemicalIndustries, Ltd.) can be named. Polyepoxide compounds (b-1) can contain3-30, preferably 3-15, recurring units of above formula (5), permolecule.

Epoxy Resin (b-2):

Polyepoxide polymer having the recurring units of a formula (6)

-   -   [in which R₇ is hydrogen or methyl]        and a number-average molecular weight of 3,000, in particular,        4,000. This polymer can be prepared, for example, by        polymerizing at least one monomer of the following formula (7)    -   [in which R₇ is hydrogen or methyl]        or at least one of such monomers with another polymerizable        monomer.

As examples of the monomers of the formula (7),3,4-epoxycyclohexylmethyl acrylate, 3,4-epoxycyclohexylmethylmethacrylate and the like can be named. As commercial products, forexample, CYCLOMER A400 and CYCLOMER M100 (tradenames, Daicel ChemicalIndustries, Ltd.) can be named.

Epoxy Resin (b-3):

Epoxy resins represented by the following formula (8)

-   -   [in the formula,        -   R₁ and R₂ are same or different, and each stands for            hydrogen, C₁-C₈ alkyl, aryl, aralkyl or halogen; R₃ stands            for hydrogen, C₁-C₁₀ alkyl, aryl, aralkyl, allyl or halogen;            R₄ and R₅ are same or different and each stands for            hydrogen, C₁-C₄ alkyl or glycidyloxyphenyl; R₅ stands for            hydrogen, C₁-C₁₀ alkyl, aryl, aralkyl, allyl or halogen; and            n is an integer of 1-38].

In the above formula (8), “alkyl” is of linear or branched chain,examples of which including methyl, ethyl, ethyl, propyl, isopropyl,butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isoamyl, hexyl, heptyl,octyl, 2-ethylhexyl, nonyl and decyl. “Aryl” may be either monocyclic orpolycyclic, examples of which including phenyl and naphthyl, phenylbeing particularly preferred. “Aralkyl” signifies aryl-substitutedalkyl, examples of which including benzyl and phenethyl, benzyl beingparticularly preferred.

“Halogen” includes fluorine, chlorine, bromine and iodine.

Furthermore, “glycidyloxyphenyl” standing for R₄ and/or R₆ of theformula (8) is a group represented by the following formula (9):

-   -   [wherein W stands for hydrogen or C₁-C₁₀ alkyl].

In the above formula (8), as R₁ and R₂, hydrogen, methyl, chlorine andbromine are convenient, hydrogen, methyl and bromine being particularlypreferred. As R₃ and R₅, methyl, tert-butyl, nonyl, phenyl, chlorine andbromine are preferred, methyl, tert-butyl, phenyl and bromine beingparticularly advantageous. Furthermore, R₄ and R₆ are preferablyhydrogen, and particularly preferred n is 1-8.

It is generally preferred for the polyepoxide compound (b-3) to have anumber-average molecular weight within a range of about 400-about 8,000,in particular, 600-2,000.

The polyepoxide compound (b-3) can be those known per se, for example,those disclosed in JP Hei 5(1993)-295321A, JP Hei 6(1994)-122850A and JPHei 6(1994)-248203A can be used. As specific examples of commercialproducts, polyglycidyl etherified cresol novolak resins such as EPICRONN-695 (tradename, Dainippon Ink & Chemicals, Inc.) and ESCN-195-XL(tradename, Sumitomo Chemical Co.) can be named.

The use rate of above-described epoxy resin (b) is suitably variableaccording to the kind of individual resins. In general terms, the baseresin (a) can be within a range of 50-90 mass parts, preferably 60-85mass parts, inter alia, 65-80 mass parts; and the epoxy resin (b), 50-10mass parts, preferably 40-15 mass parts, inter alia, 35-20 mass parts,per 100 mass parts of combined solid content of the base resin (a) andepoxy resin (b).

The electrodeposition paint (A) according to the present invention maycontain, besides above-described epoxy resin (b), other curing agentknown per se. As such concurrently useful curing agent, for example,blocked polyisocyanate compound which is an addition reaction product ofpolyisocyanate compound and isocyanate blocking agent can be named.

As the polyisocyanate compound, for example, aromatic, alicyclic oraliphatic polyisocyanate compounds such as tolylene diisocyanate,xylylene diisocyanate, phenylene diisocyanate,bis(isocyanatomethyl)cyclohexane, tetramethylene diisocyanate,hexamethylene diisocyanate, methylene diisocyanate, isophoronediisocyanate and the like; and end-isocyanate-containing prepolymerswhich are obtained by reacting excessive amount of these isocyanatecompounds with low molecular weight, active hydrogen-containingcompounds such as ethylene glycol, propylene glycol, trimethylolpropane,hexanetriol, castor oil and the like can be named.

The isocyanate blocking agent adds to isocyanate groups in thepolyisocyanate compounds to block the same. It is important that theblocked polyisocyanate compounds formed upon the addition should bestable at normal temperature but when they are heated to above theirdissociation temperature, the blocking agent be dissociated toregenerate free isocyanate groups.

In particular, in order to make the heat loss (cf. note 1) inelectrocoated film of electrodeposition paint (A) under 170° C.-20minutes' heating not more than 5% by weight, it is preferred to use lowmolecular weight compound having a molecular weight not higher than 130as the blocking agent. As specific examples, phenolic blocking agentsuch as phenol, cresol, xylenol, chlorophenol, ethylphenol and the like;lactam blocking agent such as ε-caprolactam, δ-valerolactam,γ-butyrolactam, β-propiolactam and the like; active methylene blockingagent such as ethyl acetoacetate, acetyl acetone and the like; alcoholblocking agent such as methanol, ethanol, propanol, butanol, amylalcohol, ethylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monobutyl ether, diethylene glycol monomethylether, propylene glycol monomethyl ether, benzyl alcohol, methylglycolate, butyl glycolate, diacetone alcohol, methyl lactate, ethyllactate and the like; oxime blocking agent such as formamidoxime,acetaldoxime, acetoxime, methylethylketoxine, diacetylmonoxime,cyclohexanoxime and the like; mercaptan blocking agent such as butylmercaptan, hexyl mercaptan, t-butyl mercaptan, thiophenol,methylthiophenol, ethylthiophenol and the like; acid amide blockingagent such as acetic acid amide, benzamide and the like; imide blockingagent such as succinimide, maleimide and the like; amine blocking agentsuch as xylidine, aniline, butylamine, dibutylamine and the like;imdazole bloking agent such as imidazole, 2-ethylimidazole and the like;and imine blocking agent such as ethyleneimine, propyleneimine and thelike can be named. Of these, oxime blocking agent such as methylethylketoxime and the like are particularly convenient for well balancedpaint stability and coating film curability.

The use rate of these blocked polyisocyanate compounds is suitablyvariable according to their kind. In general terms, the base resin (a)can be used within a range of 50-90 mass parts, preferably 60-85 massparts, inter alia, 65-80 mass parts; the epoxy resin (b), 35-5 massparts, preferably 28-12 mass parts, inter alia, 25-19 mass parts; andthe blocked polyisocyanate compound, 15-5 mass parts, preferably 12-3mass parts, inter alia, 10-1 mass parts; per 100 mass parts of combinedsolid content of the three components, i.e., the base resin (a), epoxyresin (b) and blocked polyisocyanate compound.

It is furthermore preferred that the electrodeposition paint (A) shouldcontain a catalyst, for improving low temperature curability of thecoating film. As the catalyst, hydroxides of metallic elements of atomicnumbers ranging 25-30 or 40-42, i.e., hydroxides of at least one metalselected from Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb and Mo can be named. Inparticular, copper (II) hydroxide, cobalt hydroxide and zinc hydroxideare preferred.

The electrodeposition paint (A) can also suitably contain metal salt ofcarboxylic acid as a curing catalyst. As the metal salt of carboxylicacid, for example, bismuth carboxylate, iron carboxylate, titaniumcarboxylate, vanadium carboxylate, zirconium carboxylate, calciumcarboxylate, potassium carboxylate, barium carboxylate, manganesecarboxylate, cerium carboxylate, aluminum carboxylate and the like canbe named. Of these, bismuth carboxylate and zirconium carboxylate areparticularly preferred, specific examples including bismuth (III)octanoate, bismuth (III) 2-ethylhexanoate, bismuth (III) oleate, bismuth(III) neodecanoate, bismuth (III) versate, bismuth (III) naphthenate,zirconyl (IV) 2-ethylhexanoate, zirconyl (IV) versate, zirconyl (IV)oleate, zirconyl (IV) naphthenate and the like can be named. Of these,bismuth (III) octanoate is particularly preferred for improvingcurability and corrosion resistance.

As the use rate (solid content) of such a catalyst, generally a range of0.1-20 wt %, in particular, 0.3-10 wt %, inter alia, 0.1-5 wt %, basedon the combined solid weight of the base resin (a) and epoxy resin (b)is preferred in respect of paint stability.

The electrodeposition paint (A) may further contain imidazole compoundas a curing catalyst. As the imidazole compound, those having amolecular weight of 68-300 per imidazole ring are suitable, i.e., wherethe compound contains one imidazole ring, its molecular weight isdesirably within a range of 68-300 and when it contains, for example,two imidazole rings, within a range of 136-600.

Specific examples of the imidazole compound include compounds having oneimidazole ring per molecule, such as imidazole, 2-methylimidazole,2-ethyl-4-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole,2-phenylimidazole, 2-phenyl-4-methylimidazole,1-benzyl-2-methylimidazole, 2-ethylimidazole, 2-isopropylimidazole,2-phenyl-4-benzylimidazole, 1-cyanoethyl-2-methylimidazole,1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-undecylimidazole,1-cyanoethyl-2-isopropylimidazole, 1-cyanoethyl-2-phenylimidazole,2,4-diamino-6-[2′-methylimidazolyl-(1)′]-ethyl-s-triazine,2,4-diamino-6-[2′-ethyl-4-methylimidazolyl-(1)′]-ethyl-s-triazine,2,4-diamino-6-[2′-undecylimidazolyl-(1)′]-ethyl-s-triazine,2-methylimidazolium-isocyanuric acid adduct,2-phenylimidazolium-isocyanuric acid adduct,1-aminoethyl-2-methylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole,2-phenyl-4-methyl-5-hydroxymethylimidazole,2-phenyl-4-benzyl-5-hydroxymethylimidazole and the like; and compoundscontaining 2 or more imidazole rings per molecule which are obtained bydehydrating above-named hydroxymethyl-containing imidazole compoundssuch as 2-phenyl-4,5-dihydroxymethylimidazole,2-phenyl-4-methyl-5-hydroxymethylimidazole and2-phenyl-4-benzyl-5-hydroxymethylimidazole; and condensing them bydeformaldehyde reaction, e.g.,4,4′-methylene-bis-(2-ethyl-5-methylimidazole) and the like, can benamed.

These imidazole compounds can be blended in the electrodeposition paint(A) at a ratio generally within a range of 0.01-10 wt %, preferably0.05-5 wt %, inter alia, 0.1-3 wt %, based on the combined solid weightof the base resin (a) and epoxy resin (b).

Corrosion resistance of the coating film can be further improved byblending inorganic bismuth compound in the electrodeposition paint (A).As the useful inorganic bismuth compound, for example, basic bismuchcarbonate, bismuth oxide carbonate, bismuth nitrate, bismuth hydroxidenitrate, basic bismuth nitrate, bismuth oxide, bismuth hydroxide andbismuth sulfate can be named, bismuth hydroxide being particularlypreferred.

Blend ratio of such an inorganic bismuth compound can be generallywithin a range of 0.1-20 wt %, preferably 0.5-10 wt %, inter alia, 1-5wt %, based on the combined solid weight of the base resin (a) and epoxyresin (b).

When blocked polyisocyanate compound is concurrently used in addition tothe epoxy resin (b) as the crosslinking agent, the paint may furthercontain tin compound as a curing catalyst. As the tin compound, forexample, organotin compound such as dibutyltin oxide, dioctyltin oxideand the like; and aliphatic or aromatic carboxylates of dialkyltin suchas dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin diacetate,dioctyltin dibenzoate, dibutyltin dibenzoate and the like can be named.

The electrodeposition paint (A) may further contain, where necessary,for example, coloring pigment such as titanium oxide, carbon black andthe like; extender such as clay, baryta, calcium carbonate, silica andthe like; and anti-rust pigment such as zinc phosphate, iron phosphate,zinc flower and the like. In particular, it is found that use of arutile-type fine particulate titanium dioxide composition formed bycoating particulate surfaces of the rutile-type fine particles oftitanium dioxide with 0.5-8.0 wt % (based on TiO₂) of zirconium oxide interms of ZrO₂ (which composition may hereafter be referred to aszirconium-coated titanium white) as the pigment can improve appearanceof the multi-layered coating film formed by the method of thisinvention.

The zirconium-coated titanium white content in the electrodepositionpaint (A) can generally be within a range of 0.1-100 mass parts,preferably 3-50 mass parts, inter alia, 5-30 mass parts, per 100 massparts of the combined solid content of the base resin (a) and epoxyresin (b), from the standpoint of appearance of the multi-layeredcoating film and paint stability.

Again, of the above extenders, use of flat pigment particles can improvechipping resistance of the multi-layered coating film. Flat pigmentparticles signify thin and flat-shaped pigment like scales, which areassumed to laminarly overlap each other with other pigment particles ina coating film to function to alleviate internal stress or externalstress. As specific examples, talc, aluminium flake, mica flake and thelike can be named.

Pigment content (note 4) of the electrodeposition paint (A) can begenerally within a range of 5-30 wt %, preferably 10-25 wt %, interalia, 15-less than 23 wt % The method of the present invention isapplicable even in environments easily causing cissing, because theelectrodeposition paint (A) allows to secure appearance of coatedsurfaces even when it has high pigment content ranging 20-30 wt %.

-   -   (Note 4) Pigment content is calculated by the following        equation: $\begin{matrix}        {{{Pigment}\quad{content}\quad(\%)} = {\frac{{weight}\quad{of}\quad{pigment}{\quad\quad}{component}}{{solid}\quad{content}\quad{of}\quad{electrodeposition}\quad{paint}} \times 100}} & \quad        \end{matrix}$    -   [in the equation, the weight of the pigment component signifies        the weight of the ash content remaining after heating a pigment        dispersion paste at 800-1,000° C. for 180 minutes, and the solid        content of the electrodeposition paint signifies the weight of        the residue remaining after heating 2 g of an electrodeposition        paint at 105° C. for 3 hours to volatilize the water and organic        solvent off].

Where necessary, the electrodeposition paint (A) can further containalcoholic or ether organic solvent, pigment dispersant such as tertiaryamino-containing acid-neutralization type epoxy resin or onium salt typeepoxy resin, surface regulating agent, surfactant, and neutralizingagent such as acetic acid or formic acid.

The electrodeposition paint (A) can be formulated following customarymethod, by adding a pigment dispersion paste to an emulsion comprising abase resin (a), epoxy resin (b), catalyst and so on, and diluting thesystem with an aqueous medium.

Thus formulated electrodeposition paint (A) can be applied ontosubstrates as earlier described, by electrocoating.

Electrocoating of the electrodeposition paint (A) can be generallyperformed using an electrocoating bath in which an electrodepositionpaint (A) is diluted with deionized water or the like to a solidconcentration of about 5-about 40 wt % and the pH is adjusted to fallwithin a range of 3.0-9.0, under such conditions of the bath temperaturenormally ranging 15-35° C. and applied voltage of 10-400 V.

The thickness of the coating film formed of the electrodeposition paint(A) is not particularly limited, but generally preferred range is 10-40μm in terms of the cured film. The electrocoated film can be washed withwater such as UF filtrate, water for industrial use or pure water.Suitable baking temperature of the coating film generally ranges about120-about 200° C., preferably about 140-about 180° C., as the surfacetemperature of the coated substrate, and the baking time can range about5-about 60 minutes, preferably about 10-about 30 minutes.

First Coloring Paint (B)

According to the method of the present invention, the first coloringpaint (B) is applied onto the cured coating film of theelectrodeposition paint (A) which is formed as described in the above.As the first coloring paint (B), either of organic solvent-basedcoloring paint or water-based coloring paint can be used, while use ofwater-based coloring paint is preferred for reducing volatile organiccompound which is one of the objects of the present invention. Hereafterwater-based coloring paint is explained.

As the base resin for first coloring paint (B), polyester resin, acrylicresin, urethane resin, alkyd resin, epoxy resin and the like, which havehydrophilic groups (e.g., carboxyl, hydroxyl, methylol, amino,sulfonate, polyoxyethylene bond and the like) of an amount sufficient tomake the resin water-soluble or water-dispersible; and functional groups(e.g. hydroxyl) which are crosslinking reactable with crosslinkingagent, can be named.

As the polyester resin, those obtained by polycondensation reaction ofat least one polybasic acid selected from alicyclic polybasic acids andother polybasic acids, with at least one polyhydric alcohol selectedfrom alicyclic polyhydric alcohols and other polyhydric alcohols. Ofthese polyester resins, those which are obtained using alicyclicpolybasic acid and/or alicyclic polyhydric alcohol as the essentialreacting components have the effect of improving chipping resistance,when they are used as the base resin.

Such alicyclic polybasic acid include compounds having at least onealicyclic structure (mainly 4- to 6-membered ring) and at least twocarboxyl groups per molecule. As specific examples,cyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid,hexahydrophthalic acid, hexahydroisophthalic acid, hexahydroterephthalicacid, hexahydrotrimellitic acid, methylhexahydrophthalic acid andanhydrides of these acids can be named. Of these,cyclohexane-1,4-dicarboxylic acid is particularly preferred.

Other polybasic acids include compounds having at least two carboxylgroups per molecule. Specific examples include phthalic acid,isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid,4,4′-diphenyldicarboxylic acid, diphenylmethane-4,4′-dicarboxylic acid,succinic acid, adipic acid, azelaic acid, sebacic acid, HET acid, maleicacid, fumaric acid, itaconinc acid, trimellitic acid, pyromellitic acidand anhydrides thereof.

Alicyclic polyhydric alcohol includes the compounds having at least onealicyclic structure (mainly 4- to 6-membered ring) and at least twohydroxyl groups per molecule, specific examples including1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol,tricyclodecanedimethanol, hydrogenated bisphenol A, hydrogenatedbisphenol F, spiroglycol, dihydroxymethyltricyclodecane and the like. Ofthese, particularly 1,4-cyclohexanedimethanol is preferred.

Among other polyhydric alcohols, as those having two hydroxyl groups permolecule, for example, glycols such as ethylene glycol, propyleneglycol, diethylene glycol, trimethylene glycol, tetraethylene glycol,triethylene glycol, dipropylene glycol, 1,4-butanediol, 1,3-butanediol,2,3-butanediol, 1,2-butanediol, 3-methyl-1,2-butanediol,1,2-pentanediol, 1,5-pentanediol, 1,4-pentanediol, 2,4-pentanediol,2,3-dimethyltrimethylene glycol, tetramethylene glycol,3-methyl-4,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol,1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 2,5-hexanediol,neopentyl glycol, hydroxypivalic acid-neopenthyl glycol ester and thelike; polylactonediols formed by adding lactones such as ε-caprolactoneto these glycols; and polyester diols such asbis(hydroxyethyl)terephthalate can be named. Also as polyhydric alcoholshaving three or more hydroxyl groups per molecule, for example,glycerine, trimethylolpropane, trimethylolethane, diglycerine,triglycerine, 1,2,6-hexanetriol, pentaerythritol, dipentaerythritol,tripentaerythritol, sorbitol, mannitol and the like can be named.

The content(s) of alicyclic polybasic acid and/or alicyclic polyhydricalcohol in the polyester resin is generally within a range of 20-70 wt%, in particular, 30-60 wt %, inter alia, 35-50 wt %, based on the totalweight of the monomers constituting the polyester resin, from theviewpoint of improving chipping resistance.

Above polyester resin can generally have a weight-average molecularweight ranging 1,000,000, preferably 2,000; a hydroxyl value ranging30-200 mgKOH/g, preferably 50-180 mgKOH/g; and an acid value ranging5-100 mgKOH/g, preferably 10-60 mgKOH/g.

As the acrylic resin, those produced by copolymerizinghydroxyl-containing radical-polymerizable unsaturated monomers and otherradical-polymerizable unsaturated monomers by customary method (e.g.,solution polymerization, emulsion polymerization and the like) can benamed. The resulting acrylic resin can generally have a number-averagemolecular weight ranging 1,000, in particular, 2,000; a hydroxyl valueranging 20-200 mgKOH/g, in particular, 50-150 mgKOH/g; and an acid valueranging 3-100 mgKOH/g, in particular, 20-70 mgKOH/g.

As the hydroxyl-containing, radical-polymerizable unsaturated monomer,for example, besides hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hyroxypropyl(meth)acrylate, hydroxybutyl (meth)acrylate and the like, PLACCEL FM1,PLACCEL FM2, PLACCEL FM3, PLACCEL FA1, PLACCEL FA2, PLACCEL FA3(tradename, Daicel Chemical Industries Ltd., caprolactone-modified(meth)acrylic acid hydroxyesters) can be used.

As other radical-polymerizable unsaturated monomers, for example,carboxyl-containing radical-polymerizable unsaturated monomers such asacrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleicacid, fumaric acid and the like; C₁-C₂₂ alkyl or cycloalkyl esters of(meth)acrylic acid such as methyl (meth)acrylate, ethyl (meth)acrylate,propyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, octyl(meth)acrylate, lauryl (meth)acrylate, 2-ethylhexyl (meth)acrylate,cyclohexyl (meth)acrylate; aromatic vinyl monomers such as styrene;(meth)acrylamide such as (meth)acrylic acid amide, N-butoxymethyl(meth)acrylamide, N-methylol (meth)acrylamide and derivatives thereof;and (meth)acrylonitrile can be named.

Above-described polyester resin or acrylic resin may be usedconcurrently with “urethane-modified polyester resin” or“urethane-modified acrylic resin” which are prepared by extendingpolyisocyanate compound from a part of hydroxyl groups in the resin byurethanation reaction to impart to the resin high molecular weight.

As the polyisocyanate compound, for example, aliphatic polyisocyanatessuch as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate,dimeric acid diisocyanate, lysine diisocyanate and the like; biuret-typeadducts and isocyanurate ring adducts of these polyisocyanates;alicyclic diisocyanates such as isophorone diisocyanate,4,4′-methylenebis(cyclohexylisocyanate), methylcyclohexane-2,4- or-2,6-diisocyanate, 1,3- or 1,4-di(isocyanatomethyl)cyclohexane,1,4-cyclohexane diisocyanate, 1,3-cyclopentane diisocyanate,1,2-cyclohexane diisocyanate and the like biuret-type adducts andisocyanurate ring adducts of these polyisocyanate; aromatic diisocyanatecompounds such as xylylene diisocyanate, metaxylylene diisocyanate,tetramethylxylylene diisocyanate, tolylene diisocyanate,4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate,1,4-naphthalene diisocyanate, 4,4′-toluydine diisocyanate, 4,4′-diphenylether diisocyanate, m- or p-phenylene diisocyanate, 4,4′-biphenylenediisocyanate, 3,3′-dimethyl-4,4′-biphenylene diisocyanate,bis(4-isocyanatophenyl)-sulfone, isopropylidenebis (4-phenylisocyanate)and the like and biuret type adducts and isocyanurate ring adducts ofthese polyisocyanates; polyisocyanates having three or more isocyanategroups per molecule, such as triphenylmethane-4,4′,4″-triisocyanate,1,3,5-triisocyanato-benzene, 2,4,6-triisocyanatotoluene,4,4′-dimethyldiphenylmethane-2,2′,5,5′-tetraisocyanate and the like andbiuret type adducts and isocyanurate ring adducts of thesepolyisocyanates; and urethanation adducts formed by reacting hydroxylgroups of polyols such as ethylene glycol, propylene glycol,1,4-butylene glycol, dimethylolpropionic acid, polyalkylene glycol,trimethylolpropane, hexanetriol and the like with polyisocyanatecompounds at such ratios as will render isocyanate groups excessive, andbiuret type adducts and isocyanurate ring adducts of thesepolyisocyanates; can be named.

These base resins can be rendered water-soluble or water-dispersible,for example, by neutralization with basic substance or acid, dependingon the kind of existing hydrophilic groups. It is also possible to makethe base resins water-soluble or water-dispersible, by emulsionpolymerization of monomeric components in the presence of surfactant orwater-soluble high molecular weight compound, in the occasion ofpreparing the base resins by polymerization.

As the crosslinking agent which may be contained in the first coloringpaint, for example, melamine resin and blocked polyisocyanate compoundcan be named. As the melamine resin, for example, methylolated melamineresin formed by methylolating melamine with formaldehyde; alkylatedmelamine resin formed by etherifying the methylol group with monohydricalcohol; methylolated or alkylated melamine resin having imino groups;and the like can be named. Mixed alkylated melamine resin obtained byusing two or more monohydric alcohols in the occasion of etherifying themethylol groups can also be used. As useful monohydric alcohol, forexample, methyl alcohol, ethyl alcohol, n-propyl alcohol, i-propylalcohol, n-butyl alcohol, i-butyl alcohol, 2-ethylbutanol,2-ethylhexanol and the like can be named.

As specific melamine resins, methylated melamine resin, imino-containingmethylated melamine resin, methylated-butylated melamine resin,imino-containing methylated-butylated melamine resin and the like arepreferred, imino-containing methylated melamine resin being particularlypreferred.

As commercially available products of these melamine resins, forexample, Cymel 202, Cymel 232, Cymel 235, Cymel 238, Cymel 254, Cymel266, Cymel 267, Cymel 272, Cymel 285, Cymel 301, Cymel 303, Cymel 325,Cymel 327, Cymel 350, Cymel 370, Cymel 701, Cymel 703, Cymel 736, Cymel738, Cymel 771, Cymel 1141, Cymel 1156, Cymel 1158, and the like(tradename, Nihon Cytec Industries, Inc., Ltd.); U-Van 120, U-Van 20HS,U-Van 2021, U-Van 2028, U-Van 2061 and the like (tradename, MitsuiChemicals, Inc.); Melan 522 and the like (tradename: Hitachi Chemical)and the like, can be named.

Blocked polyisocyanate compounds are polyisocyanates having at least twofree isocyanate groups per molecule, whose isocyanate groups are blockedwith blocking agent. As the polyisocyanate compounds, for example,aliphatic polyisocyanates such as hexamethylene diisocyanate,trimethylhexamethylene diisocyanate, dimeric acid diisocyanate, lysinediisocyanate and biuret type adducts or isocyanurate ring adducts ofthese polyisocyanates; alicyclic diisocyanates such as isophoronediisocyanate, 4,4′-methylenebis-(cyclohexylisocyanate),methylcyclohexane-2,4-(or-2,6-)diisocyanate, 1,3-(or1,4-)di(isocyanatomethyl)cyclohexane, 1,4-cyclohexane diisocyanate,1,3-cycopentane diisocyanate, 1,2-cyclohexane diisocyanate and biurettype adducts or isocyanurate ring adducts of these polyisocyanates;aromatic diisocyanates such as xylylene diisocyanate, meta-xylylenediisocyanate, tetramethylxylylene diisocyanate, tolylene diisocyanate,4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate,1,4-naphthalene diisocyanate, 4,4′-toluydine diisocyanate,4,4′-diphenylether diisocyanate, (m- or p-)phenylene diisocyanate,4,4′-biphenylene diisocyanate, 3,3′-dimethyl-4,4′-biphenylenediisocyanate, bis(4-isocyanatephenyl)sulfone,isopropyridenebis(4-phenylisocyanate) and biuret type adducts orisocyanurate ring adducts of these polyisocyanates; polyisocyanateshaving three or more isocyanate groups per molecule such astriphenylmethane-4,4′,4″-triisocyanate, 1,3,5-triisocyanatobenzene,2,4,6-triisocyanato-toluene, 4,4′-dimethyldiphenylmethane-2,2′,5,5′-tetraisocyanate and biuret type adducts or isocyanurate ringadducts of these polyisocyanates; urethanation adducts formed byreacting polyisocyanate compounds with polyols such as ethylene glycol,propylene glycol, 1,4-butylene glycol, dimethylolpropionic acid,polyalkylene glycol, trimethylolpropane, hexanetriol and the like, atsuch ratios that the isocyanate groups are in excess of the hydroxylgroups of such polyols, and biuret type adducts and isocyanurate ringadducts of these polyisocyanates, and the like can be named.

Blocking agent is to block free isocyanate groups. Formed blockedpolyisocyanate compounds are stable at normal temperature but whenheated, for example, to 100° C. or above, preferably to 130° C. orabove, the blocking agent is dissociated to regenerate the freeisocyanate groups which can readily react with hydroxyl groups.

The blocking agent may be of glycolic acid ester type, alcohol type,oxime type, active methylene type, mercaptan type, acid amide type,amine type, imidazole type, carbamic acid ester type or sulfite type.Besides those, 3,5-dimethylpyrazole, 3-methylpyrazole,4-nitro-3,5-dimethylpyrazole and 4-bromo-3,5-dimethylpyrazole and thelike can also be used as the blocking agent.

As the blocking agent for blocking isocyanate groups of polyisocyanatecompounds, hydroxycarboxylic acid having at least one hydroxyl group andat least one carboxyl group, such as hydroxypivalic acid,dimethylolpropionic acid and the like can also be used. By neutralizingthe carboxyl group(s) in the hydroxycarboxylic acid, blockedpolyisocyanate compound imparted with water dispersibility can beobtained. As commercially available blocked polyisocyanate compounds,for example BAYHYDROL BL 5140 (tradename, Sumitomo Bayer Urethane, Ltd.)can be named.

The first coloring paint (B) can further be suitably blended withaqueous dispersion of urethane resin which includes the productsobtained by reacting aliphatic and/or alicyclic diisocyanate, at leastone diol selected from polyether diols, polyester diols andpolycarbonate diols having number-average molecular weight ranging500-5,000, low molecular weight polyhydroxy compound anddimethylolalkane acid in an aqueous medium. The urethane resin can havea number-average molecular weight (cf. note 3) generally within a rangeof 2,000, preferably 4,000, inter alia, 5,000, for favorable chippingresistance and surface smoothness of the coating film.

As commercially available aqueous dispersions of such urethane resins,for example, U-COAT UX497, U-COAT UX4300, U-COAT UX5000, U-COAT UX8100(tradename, Sanyo Chemical Industries), NEOREZ R-940, R-941, R-960,R-962, R-966, R-967, R-962, R-9603, R-9637, R-9618, R-9619, XR-9624,VONDIC 1310NSC (trademames, ICI), HYDRAN HW-310, HW-311, HW-312B,HW-301, HW-111, HW-140, HW-333, HW-340, HW-350, HW-910, HW-920, HW-930,HW-935, HW940, HW-960, HW-970, HW-980, AP-10, AP-20, AP-30, AP-40,AP-60, AP-70 and AP-60LM (tradename, Dainippon Ink & Chemicals, Inc.)can be named.

Where necessary, the first coloring paint can be further suitablyblended with coloring pigment, iridescent pigment, extender, dispersant,antisettling agent, organic solvent, urethanation reaction acceleratingcatalyst (e.g., organotin compound or the like), catalyst foraccelerating the crosslinking reaction of hydroxyl groups in the baseresin with melamine resin (e.g., acid catalyst), defoaming agent,thickener, anti-rusting agent, UV absorber, surface regulating agent andthe like.

The first coloring paint (B) can be formulated by dissolving ordispersing those components as described in the foregoing in aqueousmedium by per se known means. After adjusting the viscosity to 50seconds Ford Cup #4 at 20° C., and the solid concentration, within arange of 20-70 wt %, preferably 35-60 wt %, the paint (B) can be appliedonto cured electrocoated film of the electrodeposition paint (A).

The first coloring paint (B) can be applied by per se known means, forexample, by air spray, airless spray, electrostatic coating and thelike, and the coated film thickness can be normally within a range of10-100 μm, preferably 10-35 μm, in terms of dry coating film thickness.

The coated article is normally pre-heated either directly or indirectlyin a drying oven at about 60-about 120° C., preferably about 70-about110° C., for about 1-60 minutes or the coated surface can be set in anatmosphere of ambient temperature or from about 25° C.-about 80° C.

Second Coloring Paint (C)

According to the method of the present invention, then a second coloringpaint (C) is applied onto an uncured coating film of the first coloringpaint (B). As the second coloring paint (C), either of organicsolvent-based coloring paint or water-based coloring paint can be used,while from the standpoint of reducing volatile organic compound,water-based coloring paint is preferred.

As the second coloring paint (C), for example, those comprising baseresin such as polyester resin, acrylic resin, alkyd resin, urethaneresin, epoxy resin and the like which have crosslinkable functionalgroups such as carboxyl, hydroxyl and the like and which are similar tothose described as to the first coloring paint (B); crosslinking agentsuch as optionally blocked polyisocyanate compound, melamine resin, urearesin and the like, similar to those described as to the first coloringpaint (B); and optionally pigment, defoaming agent, thickener,anti-rusting agent, UV absorber, surface regulating agent and the like,where necessary, can be used.

The second coloring paint (C) can be applied by per se known means, forexample, by air spray, airless spray, electrostatic coating and thelike, and the coated film thickness can be normally within a range of5-40 μm, preferably 10-30 μm, in terms of dry coating film thickness.

The coating film after application can be suitably pre-heated and/orset. The pre-heating can be normally conducted by heating the coatedarticle either directly or indirectly in a drying oven at about 60-about120° C., preferably about 70-about 110° C., for about 1-60 minutes. Thesetting of the coated surface can be conducted in an atmosphere ofambient temperature or from about 25° C.-about 80° C.

Clear Paint (D)

On the coating film of the second coloring paint (C) formed as above,further a clear paint (D) is coated. As the clear paint (D), forexample, organic solvent-based or water-based clear paint (D) which arecustomarily used for coating automobile bodies can be used.

More specifically, organic solvent-based paints or water-based paintswhich contain base resin such as acrylic resin, polyester resin, alkydresin, urethane resin, epoxy resin and the like, having crosslinkablefunctional groups such as hydroxyl, carboxyl, epoxy and the like; andcrosslinking agent such as melamine resin, urea resin, optionallyblocked polyisocyanate compound, carboxyl-containing compound or resin,epoxy-containing compound or resin and the like, can be used.

The clear paint (D) may contain, where necessary, coloring pigmentand/or iridescent pigment to an extent as will not impair transparencyof the coating film. Furthermore, extender, UV absorber, and the likemay also be suitably contained.

The clear paint (D) can be applied onto the second coloring paint(C)-coated surface by the means known per se, for example, electrostaticcoating, airless spraying or air spraying, to a dry coating filmthickness within a range of 10-60 μm, preferably 25-50 μm.

Baking of Coating Film

The multi-layered coating film composed of the three layers of uncuredcoating films of the first coloring paint (B), second coloring paint (C)and clear paint (D) can be simultaneously cured by ordinary coatingfilm-baking means, for example, heating by hot air heating, IR rayheating or induction heating, at about 80-about 170° C., preferablyabout 120-about 160° C., for about 20-40 minutes, whereupon providingmulti-layered film excelling in appearance, corrosion resistance,anti-chipping property and the like.

Furthermore, also by 4-coat-1-bake system in which the electrocoatedfilm of the electrodeposition paint (A) is not baked and dried, but leftuncured and subjected to setting, air-blowing or pre-heating, and ontowhich the first coloring paint (B), second coloring paint (C) and clearpaint (D) are successively applied, and the four-layered coating filmcomposed of those of the electrodeposition paint (A), first coloringpaint (B), second coloring paint (C) and clear paint (D) is given singletime baking, better appearance and corrosion resistance than those ofthe conventional cases of using amine-added epoxy resin-blockedisocyanate crosslinking type electrodeposition paint can be secured.

EXAMPLES

Hereinafter the invention is explained more specifically, referring toworking Examples, it being understood that the invention is not limitedto these Examples only. In the Examples, “part” and “%” are by weight.

Preparation of Electrodeposition Paint

Production Example 1 Preparation of Base Resin No. 1

A flask equippel with a stirrer, thermometer, dropping funnel and areflux condenser was charged with 155 parts of EHPE-3150 (note 5), 70parts of diethanolamine and the whole amount of the following phenolichydroxyl-containing product, which were reacted at 160° C. for 5 hours.Thereafter 692 parts of methyl propanol was added to provide base resinNo. 1 having a hydroxyl equivalent of 443, amine value of 63 mgKOH/g anda solid content of 60%.

Phenolic Hydroxyl-Containing Compound:

-   -   A product obtained by mixing 475 parts of bisphenol A diglycidyl        ether having an epoxy equivalent of 190, 285 parts of bisphenol        A, 53 parts of diethanolamine and 80 parts of carbitol,        dissolving the mixture by heating and causing the reactants to        react by maintaining them at 130° C. for 3 hours.    -   (Note 5) EHPE-3150: tradename, Daicel Chemical Industries, Ltd.,        an epoxy resin having at least two epoxy-containing functional        groups per molecule on the average, in which epoxy groups are        bound to alicyclic skeletal structure: epoxy equivalent=180.

Production Example 2 Preparation of Curing Agent No. 1

Two (2) parts of azobisdimethylvaleronitrile was dissolved in 33.4 partsof CYCLOMER M100 (3,4-epoxycyclohexylmethyl methacrylate). The solutionwas dropped into a 100° C. mixed solvent of 10 parts of methyl isobutylketone and 10 parts of ethylene glycol monobutyl ether over 2 hours,aged for an hour, given a temperature raise to 125° C. and aged for anadditional hour to provide curing agent No. 1 having an epoxy equivalentof 196 and solid content of 60%.

Production Example 3 Preparation of Base Resin No. 2 (for ComparativeExample)

To 1010 parts of EPICOAT 828 EL (tradename, Japan Epoxy Resin Co., anepoxy resin), 390 parts of bisphenol A and 0.2 part ofdimethylbenzylamine were added, and reacted at 130° C. until the epoxyequivalent reached 800.

Then 74 parts of dimethylolbutyric acid, 63 parts of diethanolamine and95 parts of ketiminated product of diethylenetriamine were added andreacted at 120° C. for 4 hours, followed by addition of 330 parts ofethylene glycol monobutyl ether. Thus base resin No. 2 having an aminevalue of 43 mgKOH/g and solid content of 80% was obtained.

Production Example 4 Preparation of Curing Agent No. 2 (for ComparativeExample)

To 270 parts of COSMONATE M-200 (tradename, Mitsui Chemicals, Inc.,crude MDI), 46 parts of methyl isobutyl ketone was added and heated to70° C. Further 281 parts of diethylene glycol monobutyl ether was slowlyadded and the temperature was raised to 90° C. While maintaining thistemperature, the reaction was continued until disappearance ofabsorption by unreacted isocyanate was confirmed by infrared absorptionspectrum measurement of the samples taken with time at time intervals.Adjusting the amount of the organic solvent, curing agent No. 2 having asolid content of 60% which was a blocked polyisocyanate compound wasobtained.

Production Example 5 Preparation of Zirconium-Treated Titanium Dioxide

Into 95 parts of hydrated titanium dioxide slurry (equivalent to 10parts of TiO₂) which was obtained by hydrolyzing titanyl sulfatesolution by heating, filtering and washing according to acceptedpractice, 7.3 parts of 48% aqueous caustic soda solution was thrownunder stirring, followed by 2 hours' aging at 95° C. Then the causticsoda-treated product was washed, and into the resulting 205 parts ofslurry, 48 parts of 35% hydrochloric acid was thrown under stirring,followed by 2 hours' aging under heating at 95° C. to provide titaniasol. So obtained titania sol was filtered, washed and dried at 150° C.for 30 minutes to provide rutile type fine particulate titanium dioxidehaving an average particle size of 0.015 μm. Into 100 parts of water,9.7 parts of above fine particulate titanium dioxide was thrown andfurther 2.8 parts of an aqueous zirconium sulfate solution containing15% of zirconium sulfate as converted to ZrO₂ was thrown thereinto understirring. By neutralizing the system with 25% aqueous ammonia, 1.0% asconverted to ZrO₂ (based on TiO₂) of oxide hydrate of zirconium wasdeposited on surface of the fine particulate titanium dioxide, followedby heating at 80° C. for 30 minutes, filtration, washing, and drying at150° C. for 30 minutes. Thus obtained dry product was calcined at 500°C. for 3 hours and pulverized in an energy mill to provide 9.0 parts ofrutile type fine particulate titanium dioxide composition having anaverage particle size of 0.028 μm with their surface coated by 4.0% asconverted to ZrO₂ (based on TiO₂) of zirconium oxide.

Production Example 6 Preparation of Emulsion No. 1 (for ComparativeExample)

The base resin No. 1 as obtained in Production Example 1 (117 parts;solid content, 70 parts), 37.5 parts (solid content, 30 parts) ofEHPE-3150 (cf. note 5) having a solid content of 80% as dissolved inethylene glycol monobutyl ether, and 7 parts of 10% formic acid weremixed and stirred to homogeneity, into which 132.5 parts of deionizedwater was dropped over about 15 minutes under violent stirring, toprovide emulsion No. 1 having a solid content of 34%.

Production Example 7 Preparation of Emulsion No. 2 (for ComparativeExample)

The base resin No. 1 as obtained in Production Example 1 (117 parts;solid content, 70 parts), 37.5 parts (solid content, 30 parts) ofEHPE-3150 (cf. note 5) having a solid content of 80% as dissolved inethylene glycol monobutyl ether, 6 parts (as solid content) of NikkaOctics Bismuch (note 6) and 7 parts of 10% formic acid were mixed andstirred to homogeneity, into which 142.0 parts of deionized water wasdropped over about 15 minutes under violent stirring, to provideemulsion No. 2 having a solid content of 34%.

-   -   (Note 6) Nikka Octics Bismuth: tradename, Nippon Kagaku Sangyo        Co., Ltd., bismuth octanoate

Production Example 8 Preparation of Emulsion No. 3 (for ComparativeExample)

The base resin No. 1 as obtained in Production Example 1 (117 parts;solid content, 70 parts), 50 parts (solid content, 30 parts) of thecuring agent No. 1 as obtained in Production Example 2 and 7 parts of10% formic acid were mixed and stirred to homogeneity, into which 120parts of deionized water was dropped over about 15 minutes under violentstirring, to provide emulsion No. 3 having a solid content of 34%.

Production Example 9 Preparation of Emulsion No. 4

The base resin No. 2 as obtained in Production Example 3 (87.5 parts;solid content, 70 parts), 50 parts (solid content, 30 parts) of curingagent No. 2 as obtained in Production Example 4 and 7 parts of 10%formic acid were mixed and stirred to homogeneity, into which 149.5parts of deionized water was dropped over about 15 minutes under violentstirring to provide emulsion No. 4 having a solid content of 34%.

The blended compositions of the emulsion Nos. 1-4 are collectively shownin Table 1. TABLE 1 Production Production Production Production Example6 Example 7 Example 8 Example 9 Emulsion No. 1 No. 2 No. 3 No. 4 Basebase resin No. 1 117 117 117 Resin solid content 60% (70) (70) (70) baseresin No. 2 87.5 solid content 80% (70) Cross- EHPE-3150 (cf. note 5)37.5 37.5 linking solid content 80% (30) (30) Agent curing agent No. 150 solid content 60% (30) curing agent No. 2 50 solid content 60% (30)Catalyst Nikka Octics Bismuth (note 6) 8.5 solid content 70%  (6)Neutralizing 10% formic acid 7 7 7 7 agent Deionized water 132.5 142.0120.0 149.5 34% Emulsion 294 312 294 294 (100)  (106)  (100)  (100) 

Production Example 10 Preparation of Pigment Dispersion Paste No. 1

To 8.33 parts (solid content, 5 parts) of base resin No. 1 as obtainedin Production Example 1, 4.4 parts of 10% formic acid was added, and towhich 15 parts of deionized water was added under stirring. Further 10parts of JR-600E (note 7), 1 part of Carbon MA-7 (note 8), 10 parts ofHydride PXN (note 9), 1 part of copper hydroxide, 3 parts of bismuthhydroxide and 7.3 parts of deionized water were added and mixed, anddispersed in a ball mill for 24 hours to provide pigment dispersionpaste No. 1 having a solid content of 50.0 wt %. The pigment content(cf. note 4) of the pigment dispersion paste No. 1 was 80 wt %.

-   -   (note 7) JR-600E: tradename, Tayca Corporation, titanium white;        Al₂O₃ coating, 3.8 mass %    -   (note 8) Carbon MA-7: tradename, Mitsubishi Chemical Co. Ltd.,        carbon black    -   (note 8) Hydride PXN: tradename, Georgia Kaolin Co., kaolin

Production Example 11 Preparation of Pigment Dispersion Paste No. 2

To 8.33 parts (solid content, 5 parts) of base resin No. 1 as obtainedin Production Example 1, 4.4 parts of 10% formic acid was added, and towhich 15 parts of deionized water was added under stirring. Further 10parts of JR-603 (note 10), 1 part of Carbon MA-7 (cf. note 8), 10 partsof Hydride PXN (cf. note 9), 1 part of copper hydroxide, 3 parts ofbismuth hydroxide and 7.3 parts of deionized water were added and mixed,and dispersed in a ball mill for 24 hours to provide pigment dispersionpaste No. 2 having a solid content of 50.0 wt %. The pigment content(cf. note 4) of the pigment dispersion paste No. 2 was 80 wt %.

(note 10) JR-603: tradename, Tayca Corporation, titanium white, ZrO₂ 0.5mass %, Al₂O₃ coating, 4.6 mass %.

Production Example 12 Preparation of Pigment Dispersion Paste No. 3

To 8.33 parts (solid content, 5 parts) of the base resin No. 1 asobtained in Production Example 1, 4.4 parts of 10% formic acid wasadded, and to which 15 parts of deionized water was added understirring. Further 10 parts of zirconium-treated titanium dioxide asobtained in Production Example 5, 1 part of Carbon MA-7 (cf. note 8), 10parts of Hydride PXN (cf. note 9), 1 part of copper hydroxide, 3 partsof bismuth hydroxide and 7.3 parts of deionized water were added andmixed, and dispersed in a ball mill for 24 hours to provide pigmentdispersion paste No. 3 having a solid content of 50.0 wt %. The pigmentcontent (cf. note 4) of the pigment dispersion paste No. 3 was 80 wt %.

Production Example 13 Preparation of Pigment Dispersion Paste No. 4

To 8.33 parts (solid content, 5 parts) of the base resin No. 1 asobtained in Production Example 1, 4.4 parts of 10% formic acid wasadded, and to which 15 parts of deionized water was added understirring. Further 10 parts of JR-600E (cf. note 7), 1 part of CarbonMA-7 (cf. note 8), 7 parts of Hydride PXN (cf. note 9), 3 parts of TalcMV (note 11), 1 part of copper hydroxide, 3 parts of bismuth hydroxideand 7.3 parts of deionized water were added and mixed, and dispersed ina ball mill for 24 hours to provide pigment dispersion paste No. 4having a solid content of 50.0 wt %. The pigment content (cf. note 4) ofthe pigment dispersion paste No. 4 was 80 wt %.

(note 11) Talc MV: tradename, United Siera Divi. Co.; talc

Production Example 14 Preparation of Pigment Dispersion Paste No. 5

To 8.33 parts (solid content, 5 parts) of the base resin No. 1 asobtained in Production Example 1, 4.4 parts of 10% formic acid wasadded, and to which 15 parts of deionized water was added understirring. Further 10 parts of JR-600E (cf. note 7), 1 part of CarbonMA-7 (cf. note 8), 10 parts of Hydride PXN (cf. note 9), 3 parts ofbismuth hydroxide and 6.3 parts of deionized water were added and mixed,and dispersed in a ball mill for 24 hours to provide pigment dispersionpaste No. 5 having a solid content of 50.0 wt %. The pigment content(cf. note 4) of the pigment dispersion paste No. 5 was 80 wt %.

Production Example 15 Preparation of Pigment Dispersion Paste No. 6

To 8.33 parts (solid content, 5 parts) of the base resin No. 1 asobtained in Production Example 1, 4.4 parts of 10% formic acid wasadded, and to which 15 parts of deionized water was added understirring. Further 10 parts of JR-600E^((cf. note 7)), 1 part of CarbonMA-7 (cf. note 8), 10 parts of Hydride PXN (cf. note 9), 3 parts ofbismuth hydroxide 1 part of dioctyltin oxide and 7.3 parts of deionizedwater were added and mixed, and dispersed in a ball mill for 24 hours toprovide pigment dispersion paste No. 6 having a solid content of 50.0 wt%. The pigment content (cf. note 4) of the pigment dispersion paste No.6 was 80 wt %.

The blended compositions of the pigment dispersion paste Nos. 1-6 ofProduction Examples 10-15 are collectively shown in Table 2. TABLE 2Production Production Production Production Production ProductionExample 10 Example 11 Example 12 Example 13 Example 14 Example 15Pigment Dispersion Paste No. 1 No. 2 No. 3 No. 4 No. 5 No. 6Pigment-Dispersing solid content 80% 8.33 8.33 8.33 8.33 8.33 8.33 Resinbase resin No. 1  (5)  (5)  (5)  (5)  (5)  (5) Neutralizer 10% formicacid 4.4 4.4 4.4 4.4 4.4 4.4 deionized water 15 15 15 15 15 15 JR-600E(cf. note 7) 10 10 10 10 JR-603 (cf. note 10) 10 Zirconium-treatedtitanium 10 dioxide Carbon MA-7 (note 8) 1 1 1 1 1 1 Hydride PXN (note9) 10 10 7 7 10 10 Talc MV (note 11) 3 3 copper hydroxide 1 1 1 1bismuth hydroxide 3 3 3 3 3 3 dioctyltin oxide 1 deionized water 7.3 7.37.3 7.3 6.3 7.3 50% Pigment Dispersion Paste 60 60 60 60 58 60 (30) (30)(30) (30) (29) (30) Pigment Content % cf. note 4) 80 80 80 80 79 80

Production Example 16 Preparation of Electrodeposition Paint No. 1

To 294 parts (solid content, 100 parts) of 34% emulsion No. 1 asobtained in Production Example 6, 60 parts (solid content, 30 parts) of50% pigment dispersion paste No. 1 as obtained in Production Example 10and 296 parts of deionized water were added to provide electrodepositionpaint No. 1 having a solid content of 20%. The pigment content (cf. note4) of the electrodeposition paint No. 1 was 18.5%.

Production Examples 17-24 Preparation of Electrodeposition Paint Nos.2-9

Electrodeposition paint Nos. 2-9 were prepared each having the blendedcomposition as shown in the following Table 3, in the same manner asProduction Example 16. TABLE 3 Production Production ProductionProduction Production Production Production Production ProductionExample Example Example Example 16 Example 17 Example 18 Example 19Example 20 Example 21 22 23 24 Electrodeposition Paint No. 1 No. 2 No. 3No. 4 No. 5 No. 6 No. 7 No. 8 No. 9 Paint Emulsion No. 1 294 294 294 294294 Blend (100) (100) (100) (100) (100)   Emulsion No. 2 312 (106)Emulsion No. 3 294 (100) Emulsion No. 4 294 294 (100) (100)   PigmentDispersion 60 60 90.4 Paste No. 1  (30)  (30)  (45.2) Pigment Dispersion60 Paste No. 2  (30) Pigment Dispersion 60 Paste No. 3  (30) PigmentDispersion 60 Paste No. 4  (30) Pigment Dispersion 58 Paste No. 5  (29)Pigment Dispersion 60 90.4 Paste No. 6  (30)  (45.2) Deionized Water 296590 296 296 617 296 341.6 296 341.6 20% Bath 650 650 650 650 675 650 726650 726 (130) (130) (130) (130) (135) (130) (145.2) (130) (145.2)Pigment Content (%) of 18.5 18.5 18.5 18.5 18.5 18.5 25 18.5 25Electrodeposition Paint (note 4)

Production Example 25 Preparation of Aqueous Acrylic Resin Dispersion

A four-necked flask equipped with a stirrer, thermometer, reflux tubeand a nitrogen-inlet tube was charged with 40 parts of deionized waterand 0.8 part of an anionic surfactant (tradename: Newcol 707SF, JapanNyukazai Co. Ltd.; non-volatile component=30%), which were stirred afternitrogen substitution of the atmosphere while being maintained at 82° C.Into the flask, first a mixture of 5 parts of an emulsified “monomericmixture” as identified in the following and 0.3 part of ammoniumperfulfate which were dissolved in 3 parts of deionized water was added,and in 20 minutes thereafter the remaining “monomeric mixture” and 0.3part of ammonium persulfate as dissolved in 3 parts of deionized waterwas dropped over 4 hours to carry out emulsion polymerization.

“Monomeric Mixture”:

-   -   A monomeric mixture formed by stirring and emulsifying 54 parts        of deionized water, 0.5 part of Newcol 707SF, 45 parts of ethyl        acrylate, 48 parts of methyl methacrylate, 5 parts of        hydroxyethyl acrylate, 1 part of acrylic acid and 1 part of        allyl methacrylate.

After the dropping was terminated, the emulsion polymerization wascontinued for further 2 hours while maintaining the temperature of 82°C., and then the temperature inside the flask was dropped to 40° C.Adjusting the pH to 8.5 with aqueous ammonia, an acrylic resin emulsionhaving a solid content of 50 wt % was obtained. The average particlesize of the emulsified resin was 0.15 μm, and the resin had an acidvalue of 7.8 mgKOH/g.

Production Example 26 Preparation of Water-Based Clear Paint

To 39 parts of the aqueous acrylic resin dispersion as obtained inProduction Example 25, 15 parts of Cymel 325 (note 12), 9 parts ofethylene glycol monobutyl ether and 1.4 parts ofN,N-dimethylaminoethanol were added and mixed by stirring. Then 3.2parts (active component, 0.8 part) of Nacure 4167 (tradename, KingIndustries Co.; a phosphoric acid-derived acid catalyst; activecomponent, 25%) was added and mixed by stirring to homogeneity, anddeionized water was slowly added under continued stirring to provide awater-based clear paint having a solid content of 40%.

-   -   (note 12) Cymel 325: tradename, an imino-containing methylated        melamine resin, Nippon Cytec Industries, Ltd.        Test Sheet

Cold-drawn steel sheet (150 mm-long, 70 mm-wide and 0.8 mm-thick) wasgiven a chemical treatment (PALBOND #3020, tradename, Nihon ParkerizingCo.; a zinc phosphate treating agent) to serve as the test sheet.

Examples and Comparative Examples Example 1 Formation of Multi-LayeredCoating Film No. 1

A multi-layered coating film No. 1 was prepared by the following steps.

Step 1: The test sheet was maintained horizontally and electrocoatedwith the electrodeposition paint No. 1. The resulting coating film washeated at 170° C. for 20 minutes, to provide a coated sheet having a 20μm-thick electrocoated film in terms of cured film thickness.

Step 2: WP-300T (tradename, Kansai Paint, a water-based first coloringpaint) was spray-coated onto the electrocoated sheet to a thickness aswould provide cured coating film thickness of 30 μm, allowed to stand atroom temperature for 3 minutes, and thereafter pre-heated at 80° C. for10 minutes.

Step 3: WBC-713T (tradename, Kansai Paint; a water-based second coloringpaint) was further spray-coated on the above coating film surface to athickness as would provide a cured coating film thickness of 15 μm,allowed to stand at room temperature for 3 minutes, and thereafterpre-heated at 80° C. for 10 minutes.

Step 4: Successively, onto the above coated surface KINO #1200 TW(tradename, Kansai Paint; an organic solvent-based clear paint) wasspray-coated to a thickness as would provide a cured coating filmthickness of 35 μm, and allowed to stand at room temperature for 5minutes. Then the three-layered coating film as formed in above Steps2-4 was baked at 140° C. for 30 minutes to provide cured multi-layeredcoating film No. 1.

Example 2-10

Multi-layered coating film Nos. 2-10 of Examples 2-10 were obtainedthrough similar operations to those of Example 1 (cured film thicknesswas same for all runs), except that the steps were changed as indicatedin the following Table 4.

The results of performance evaluation of the multi-layered coating filmsas obtained in Examples 1-10 are concurrently shown in Table 4.

Comparative Example 1

A multi-layered coating film No. 11 was prepared by the following steps.

Step 1: The test panel was maintained horizontally and electrocoatedwith the electrodeposition paint No. 8. The resulting coating film washeated at 170° C. for 20 minutes, to provide a coated sheet having a 20μm-thick electrocoated film in terms of cured film thickness.

Step 2: WP-300T (tradename, Kansai Paint; a water-based first coloringpaint) was spray-coated onto the electrocoated sheet to a thickness aswould provide cured coating film thickness of 30 μm, allowed to stand atroom temperature for 3 minutes, and thereafter pre-heated at 80° C. for5 minutes.

Step 3: WBC-713T (tradename, Kansai Paint; a water-based second coloringpaint) was further spray-coated on the above coating film surface to athickness as would provide a cured coating film thickness of 15 μm,allowed to stand at room temperature for 3 minutes, and thereafterpre-heated at 80° C. for 5 minutes.

Step 4: Successively, onto the above coated surface KINO #1200 TW(tradename, Kansai Paint; an organic solvent-based clear paint) wasspray-coated to a thickness as would provide a cured coating filmthickness of 35 μm, and allowed to stand at room temperature for 5minutes. Then the three-layered coating film as formed in above Steps2-4 was baked at 140° C. for 30 minutes to provide cured multi-layeredcoating film No. 11.

Comparative Examples 2-4

Comparative Example 1 was repeated except that the steps were changed ineach run as shown in the following Table 5, to provide multi-layeredcoating film Nos. 12-14.

The results of performance evaluation of the multi-layered coating filmsas obtained in Comparative Examples 1-4 are concurrently shown in Table5. TABLE 4 Example 1 Example 2 Example 3 Example 4 Example 5Multi-layered Coating Film No. 1 No. 2 No. 3 No. 4 No. 5 Step 3C1B 3C1B3C1B 3C1B 3C1B Step Step 1 Electrodeposition paint No. 1 No. 2 No. 3 No.4 No. 5 Baking or setting 170° C. 170° C. 170° C. 170° C. 170° C. 20min. 20 min. 20 min. 20 min. 20 min. Step 2 First coloring paint WP-300TWP300T WP-300T WP-300T WP-300T Pre-heating 80° C. 80° C. 80° C. 80° C.80° C. (temperature-time) 5 min. 5 min. 5 min. 5 min. 5 min. Step 3Second coloring paint WBC-713T WBC-713T WBC-713T WBC-713T WBC-713TPre-heating 80° C. 80° C. 80° C. 80° C. 80° C. (temperature-time) 5 min.5 min. 5 min. 5 min. 5 min. Step 4 Clear paint KINO # KINO # KINO # KINO# KINO # 1200TW 1200TW 1200TW 1200TW 1200TW Baking (temperature-time)140° C. 140° C. 140° C. 140° C. 140° C. 30 min. 30 min. 30 min. 30 min.30 min. Electrodeposited Heat loss of electrocoated film (note 14) 2.72.7 2.7 2.7 2.7 Coating Power spectral value(note 15) average 47 36 3547 40 Film value Alone integrated 1.2 × 10⁵ 9.1 × 10⁴ 8.9 × 10⁴ 1.2 ×10⁵ 1.0 × 10⁵ value Multi- Appearance of multi- (note 16) ◯

◯

layered layered coating film coating Corrosion resistance (note 17) ◯ ◯◯ ◯ ◯ film Anti-chipping property (note 18) ◯ ◯ ◯ ◯

Example 6 Example 7 Example 8 Example 9 Example 10 Multi-layered CoatingFilm No. 6 No. 7 No. 8 No. 9 No. 10 Step 3C1B 3C1B 3C1B 3C1B 4C1B StepStep 1 Electrodeposition paint No. 6 No. 7 No. 1 No. 1 No. 1 Baking orsetting 170° C. 170° C. 170° C. 170° C. 25° C. 20 min. 20 min. 20 min.20 min. 20 min. Step 2 First coloring paint WP-300T WP-300T WP-300TTP-65-2 WP-300T (note 13) Pre-heating 80° C. 80° C. 80° C. 80° C. 80° C.(temperature-time) 5 min. 5 min. 5 min. 5 min. 5 min. Step 3 Secondcoloring paint WBC-713T WBC-713T WBC-713T WBC-713T WBC-713T Pre-heating80° C. 80° C. 80° C. 80° C. 80° C. (temperature-time) 5 min. 5 min. 5min. 5 min. 5 min. Step 4 Clear paint KINO # KINO # water-based KINO #KINO # 1200TW 1200TW clear paint 1200TW 1200TW Production Example 26Baking (temperature-time) 140° C. 140° C. 140° C. 140° C. 140° C. 30min. 30 min. 30 min. 30 min. 30 min. Electrodeposited Heat loss ofelectrocoated film (note 14) 2.8 2.0 2.7 2.7 2.7 Coating Power spectralvalue(note 15) average 52 60 47 47 47 Film value Alone integrated 1.3 ×10⁵ 01.5 × 10⁵ 1.2 × 10⁵ 1.2 × 10⁵ 1.2 × 10⁵ value Multi- Appearance ofmulti- (note 16) ◯ ◯ ◯ ◯ ◯ layered layered coating film coatingCorrosion resistance (note 17) ◯

◯ ◯ ◯ film Anti-chipping property (note 18) ◯ ◯ ◯ ◯ ◯

(note 13) TP-65-2: tradename, Kansai Paint, an organic solvent basedfirst coloring paint TABLE 5 Comparative Comparative ComparativeComparative Example 1 Example 2 Example 3 Example 4 Multi-layeredCoating Film No. 11 No. 12 No. 13 No. 14 Step 3C1B 3C1B 3C2B 4C1B StepStep 1 Electrodeposition No. 8 No. 9 No. 8 No. 8 paint Baking or setting170° C. 170° C. 170° C. 25° C. 20 min. 20 min. 20 min. 20 min. Step 2First coloring paint WP-300T WP300T WP-300T WP-300T Pre-heating 80° C.80° C. 140° C. 80° C. (temperature-time) 5 min. 5 min. 30 min. 5 min.Step 3 Second coloring WBC-713T WBC-713T WBC-713T WBC-713T paintPre-heating 80° C. 80° C. 80° C. 80° C. (temperature-time) 5 min. 5 min.5 min. 5 min. Step 4 Clear paint KINO # KINO # KINO # KINO # 1200TW1200TW 1200TW 1200TW Baking 140° C. 140° C. 140° C. 140° C.(temperature-time) 30 min. 30 min. 30 min. 30 min. Electrodeposited Heatloss of electrocoated 8.1 7.8 8.1 8.1 Coating Film film (note 14) AlonePower spectral average 180 200 180 180 value (note 15) value integrated4.5 × 10⁵ 5.1 × 10⁵ 4.5 × 10⁵ 4.5 × 10⁵ value Multi-layered Appearanceof (note 16) Δ X Δ X coating film multi-layered coating film Corrosion(note 17) ◯ ◯ ◯ Δ resistance Anti-chipping (note 18) ◯ Δ ◯ Δ property

-   -   (note 14) Heat loss of electrocoated film: (cf. note 1)    -   (note 15) Power spectral value:        -   Following the measuring method as described in (note 2), the            values measured with SURFCOM 130A (tradename, Tokyo Seimitsu            Co. Ltd.) at a measuring length 50 mm and data-sampling            intervals of 10 μm    -   (note 16) Appearance of multi-layered coating film:        -   Wavescan Plus (tradename, BYK Gardner Co.) was used. While            specular gloss meters measure images, Wavescan Plus focuses            on coating film surface, which applies laser beam generated            from the measuring instrument to coating film surface and            minutely detects intensity of reflected light. Through the            intensity data of reflection light, optical unevenness of            coating film surface can be observed at a level close to            visual observation.        -   As the wavelength structures, two kinds of long wave values            (LW) of long wavelength structure and short wave values (SW)            of short wavelength structure can be measured. Lower numeral            values signify more favorable level of multi-layered coating            film appearance:        -   {circle around (•)} short wave value (SW) less than 12        -   ◯ short wave value (SW) 12-less than 15        -   Δ short wave value (SW) 15-less than 20        -   x short wave value (SW) exceeds 20.    -   (note 17) Corrosion resistance:        -   Formed multi-layered coating film was cross-cut to the depth            reaching the substrate surface, and was given a saline            solution spray test for 840 hours following JIS Z-2371.            Corrosion resistance was evaluated by the following standard            according to width of rust and blister development from the            knife cuts:    -    {circle around (•)}: the maximum width of rusting and        blistering from the cuts was less than 1.5 mm (single side);    -    ◯: the maximum width of rusting and blistering from the cuts        was no less than 1.5 mm but less than 2.5 mm (single side);    -    Δ: the maximum width of rusting and blistering from the cuts        was no less than 2.5 mm but less than 3.5 mm (single side);    -    x: the maximum width of rusting and blistering from the cuts        was 3.5 mm or more (single side).    -   (note 18) Anti-chipping property:        -   The test sheet on which a multi-layered coating film was            formed was fixed on test piece-holding stand of a chipping            tester (Flying Stone Tester, JA-400 Model, Suga Tester Co.)            with its coated surface positioned at a right angle with the            mouth of the stone nozzle. Fifty (50) g of granite rubbles            of grain size No. 7 was blown at the coated surface at −20°            C., with compressed air of 0.294 MPa (3 kgf/cm²). Thereafter            an adhesive fabric tape (Fuji Industries) was stuck on the            coated surface and rapidly peeled off. The extent of cracks            occurring on the coating film was visually observed and            anti-chipping property was evaluated according to the            following standard.            -   {circle around (•)}: The crack size was considerably                small, which occurred at a part of the second coloring                coating film.            -   ◯: The crack size was small and a part of the first                coloring coating film was exposed.            -   Δ: The crack size was large, a part of the first                coloring coating film was broken off, and the                electrodeposited coating film or the steel sheet was                exposed.            -   x: The crack size was considerably large, the first                coloring coating film was notably exposed, or the first                coloring coating film was broken off to expose the                electrodeposited coating film or the steel sheet,                markedly imparing in appearance.

1. A method of forming multi-layered coating film which comprisesapplying a first coloring paint (B), second coloring paint (C) and clearpaint (D) successively wet-on-wet, onto a cured coating film of anelectrodeposition paint (A) showing a heat loss (X) of not more than 5%by weight, said heat loss being calculated according to the followingequation:heat loss (X)=[(Y−Z)/Y]×100 [wherein Y is the weight of a dry coatingfilm remaining after removal of the water content from an uncuredcoating film obtained by electrocoating the electrodeposition paint (A),by heating at 105° C. for 3 hours; and Z is the weight of the cured filmafter heating the dry coating film at 170° C. for 20 minutes]; andheat-curing the so formed three-layered coating film simultaneously. 2.A method as set forth in claim 1, in which the heat loss (X) of theelectrodeposition paint (A) is not more than 4% by weight.
 3. A methodas set forth in claim 1, in which the electrodeposition paint (A)comprises base resin (a) obtained through reaction of epoxy resin (a₁),amine compound (a₂) and phenolic compound (a₃), and epoxy resin (b) as acrosslinking agent.
 4. A method as set forth in claim 2, in which theepoxy resin (a₁) is an epoxy resin of the following formula (1)

having at least two epoxy-containing functional groups per molecule. 5.A method as set forth in claim 2, in which the epoxy resin (a₁) has anepoxy equivalent within a range of 140-1,000 and a number-averagemolecular weight within a range of 200-50,000.
 6. A method as set forthin claim 2, in which the amine compound (a₂) is a primary or secondaryamine compound containing primary hydroxyl group(s).
 7. A method as setforth in claim 2, in which the phenolic compound (a₃) contains at leastone phenolic hydroxyl group per molecule.
 8. A method as set forth inclaim 7, in which the phenolic compound (a₃) is a bisphenolic compound.9. A method as set forth in claim 2, in which the base resin (a) has anamine value within a range of 20-150 mgKOH/g; hydroxyl value within arange of 300-1,000 mgKOH/g; and a number-average molecular weight withina range of 800-15,000.
 10. A method as set forth in claim 2, in whichthe epoxy resin (b) is polyepoxide compound containing at least twoepoxy-containing functional groups per molecule on the average, saidfunctional group being formed of epoxy group(s) binding to alicyclicskeletal structure, or glycidyl etherified product of novolak resin. 11.A method as set forth in claim 2, in which the epoxy resin (b) isselected from the group consisting of polyepoxide compounds havingrecurring units of the following formula (5)

polyepoxide polymers having recurring units of the following formula (6)

[in which R₇ is hydrogen or methyl] and a number-average molecularweight within a range of 3,000; and epoxy resins of the followingformula (8)

[in the formula, R₁ and R₂ are same or different, and each stands forhydrogen, C₁-C₈ alkyl, aryl, aralkyl or halogen; R₃ stands for hydrogen,C₁-C₁₀ alkyl, aryl, aralkyl, allyl or halogen; R₄ and R₅ are same ordifferent and each stands for hydrogen, C₁-C₄ alkyl orglycidyloxyphenyl; R₅ stands for hydrogen, C₁-C₁₀ alkyl, aryl, aralkyl,allyl or halogen; and n is an integer of 1-38].
 12. A method as setforth in claim 2, in which the electrodeposition paint (A) contains0.1-20 mass % of bismuth octanoate, based on the combined solid weightof the base resin (a) and epoxy resin (b).
 13. A method as set forth inclaim 2, in which the electrodeposition paint (A) contains a rutile typefine particulate titanium dioxide composition which is formed by coatingsurface of rutile type fine particulate titanium dioxide with 0.5-8.0%by weight (based on TiO₂) of zirconium oxide as converted to ZrO₂.
 14. Amethod as set forth in claim 1, in which the electrodeposition paint (A)forms a cured electrocoated film having an average power spectral valuenot greater than 70, said value being obtained by power spectralfrequency analysis comprising measuring surface roughness of anelectrocoated film which has been cured by heating at 170° C. for 20minutes, with a surface roughness meter over a measuring length of 50 mmat 10 μm intervals, and then Fourier converting the obtained data.
 15. Amethod as set forth in claim 1 in which the electrodeposition paint (A)forms a cured electrocoated film having an integrated spectral powervalue within a wavelength range 0.02-1 mm of not greater than 1.7×10⁵,said integrated value being obtained by power spectral frequencyanalysis comprising measuring surface roughness of the electrocoatedfilm which has been cured by heating at 170° C. for 20 minutes, with asurface roughness meter over a measuring length of 50 mm at 10 μmintervals, and then Fourier converting the obtained data.
 16. Articleson which multi-layered coating film is formed by any of the methods asdescribed in claims 1-15.